An electrochemical fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of the fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell has a membrane-electrode-assembly (MEA) that typically includes a thin, solid polymer membrane-electrolyte disposed between anode and cathode layers. The anode and cathode layers typically include a finely divided catalyst, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The anode and cathode layers are sometimes termed catalyst layers (CL). The MEA of the PEM fuel cell is typically characterized by enhanced proton conductivity under wet conditions. Proper water management and humidification are generally required for effective operation of the MEA.
The durability of the MEA strongly depends on the hydration status of the polymer electrolyte membrane used in the MEA. Under typical operating conditions, the MEA cycles through relatively wet and relatively dry states. These membrane hydration cycles are particularly prevalent during fuel cell start-up and shut-down operations and as power demand fluctuates during operation of the fuel cell. One of the consequences of the hydration cycling is a long-term degradation of the mechanical durability of the MEA.
The MEA is generally disposed between a pair of porous conductive materials, also known as gas diffusion media (GDM), which performs a multifunctional role in PEM fuel cells. For example, the GDM distribute gaseous reactants such as hydrogen and oxygen/air, to the anode and cathode layers. The GDM conducts electrons and transfers heat generated at the MEA to a coolant. With respect to water management of the fuel cell, the GDM transports water produced by the electrochemical fuel cell reaction away from the PEM. The water management capability of the GDM is critical to any optimization of fuel cell performance. Oftentimes, the GDM includes a microporous layer (MPL) that provides a transition layer between the MEA electrodes and the GDM. The MPL further assists in water transport from the MEA.
A desirable GDM both maintains membrane electrolyte hydration during dry operating conditions for effective proton conductivity and removes excess water during wet operating conditions, thus militating against flooding of the fuel cell.
As is known in the art, GDM having a spatially varying mass transport resistance may be employed for water management in electrochemical fuel cells. As described in U.S. Pat. No. 6,933,067 to Kawahara et al., a diffusion layer may be sectioned into a plurality of portions, including an upstream portion and a downstream portion. The upstream portion has a structure for preventing a drying-up of the fuel cell and the downstream portion has a structure for preventing a flooding of the fuel cell. U.S. Pat. App. Pub. No. 2005/0026018 to O'Hara et al. discloses a diffusion media and a scheme for spatially varying parameters of the diffusion media to address issues related to water management. Johnson et al. in European Pat. No. 0846347 and U.S. Pat. No. 5,840,438 report anode and cathode substrates having an in-plane, non-uniform structure that enables controlled transport of reactant toward an electrocatalyst layer and controlled transport of water away from the electrocatalyst layer.
There is a continuing need for a method of selecting diffusion media with spatially varying diffusion resistance, and to optimize fuel cell operating conditions for a desired automotive drive cycle. A method allowing for a minimized inlet relative humidity (RH), enabling drier operation of the fuel cell and employment of electrolyte membranes having a high conductivity for a given RH sensitivity, and improving freeze performance and durability, is also desired.